One arbitrary point on an upper surface of a workpiece and two points at arbitrary heights on a tapered section of the workpiece are measured using a contact detector disposed on an upper wire guide of a wire electric discharge machine to calculate the shape accuracy and tapered angle. Based on the calculated data, the upper and lower wire guide support positions of the upper and lower wire guides are calculated. Using the calculated upper and lower wire guide support positions, electric discharge machining is performed on the workpiece.